1. Field of the Invention
This invention relates to ATM (asynchronous transfer mode) transmission apparatus used in communication networks wherein digital service units (DSUs) and exchange (LT unit) having STM (synchronous transfer mode) line interfaces are connected by ATM, and more specifically to ATM transmission equipment wherewith the transmission path portion of the line interface is replaced with an ATM network, LT unit, DSU, and terminal, etc., can be accommodated, and a T-point interface is provided in the interest of reducing DSU installation costs, so that 1.5M dedicated line service, for example, can be efficiently accommodated in the ATM network.
2. Description of the Related Art
Private networks are recently being widely developed by communication businesses (carriers), and for in-house communication, utilizing the statistical multiplexing benefits and flexible network operability of ATM technology. During the transition period while this type of networking using ATM technology is proliferating, however, there are existing networks, such as communication networks using high-speed digital lines or ISDN lines. Hence a scheme is needed for handling the equipment of these existing networks.
When a carrier is to expand its network, for example, one approach is to set up an ATM network beforehand to provide lines for connecting between the carrier equipment and in-home unit such as terminals, in preparation for the transition to the future ATM network. In such cases, it is important that the ATM network be configured so that it can accommodate the preexisting station exchange (LT unit) and digital service units (DSUs) that make up the carrier equipment and the in-home unit.
In FIG. 43 is given a simplified block diagram of an ordinary conventional communication network configured with STM lines.
Specifically, in FIG. 43, in-home unit. (user terminals) 12-1 and 12-2 are connected respectively through digital service units (DSUs) 11-1 and 11-2 to the station exchange (LT unit) 10 having an:STM line interface. In this diagram, U points configure the.subscriber line interfaces (hereinafter called U point interfaces). These U-point interfaces comprise metallic wire or optical fiber and connect between the LT unit 10 and remotely located DSUs 11-1 and 11-2. T points configure so-called T point interfaces.
Now, when an ATM network is set up beforehand to provide lines for connecting carrier equipment with terminals and other in-home unit with a view to transition to the future ATM network, as described in the foregoing, one possible configuration involves installing the ATM network at the U points configuring the line interfaces in FIG. 43. In this configuration, in the first place, it must be possible to emulate the existing transmission paths, and, in the second place, it must be possible to provide T point interfaces for the in-home unit. In the prior art, however, no suitable transmission apparatus exists that satisfies these requirements.
Furthermore, in a network wherein an ATM network is provided for the line interfaces in an StM network, what is ordinarily done is to perform cell assembly and cell disassembly on all data strings (including frame F bits) on the STM lines and to transmit these into the ATM network. When this is done, however, network resources are wasted because the ATM network becomes occupied by bands having the speeds of the STM lines.
In communication systems designed so that conventional video, audio, and text data, etc., are assembled into cells or packets of fixed length and then transmitted from the transmitting equipment to the receiving equipment, when some kind of trouble develops on a line or communication path on the way from the transmitting side to the receiving side, resulting in a situation wherein the desired transmission quality can no longer be guaranteed, the measure of interrupting that line must be taken so that line or communication path is not used.
One conventional method of detecting faults in lines or communication paths, for example, is that disclosed in Japanese Patent Laid-open No. H5-63761/1993 (gazette), whereby, when transmission data are transmitted, information on the time until the next transmission data is added at the beginning thereof. If the time information can be normally verified at the receiving end, a normal reception confirmation signal is returned to the transmitting end. When the time information cannot be normally verified, a retransmit request is transmitted back to the transmitting end, or a line fault state ensues and that fact is output to the outside.
In an ATM network system, on the other hand, when a line break or synchronization fault is detected at the receiving terminal, and alarm information indicating line fault is transmitted back to the opposite end, i.e. to the transmitting equipment, an OAM cell (operation and maintenance cell) is inserted in the ATM cell flow. This alarm information is inserted into the OAM cell and transmitted to the opposite ATM exchange, whereupon, in that opposing ATM exchange, that OAM cell is-resolved, the alarm information extracted, and the resulting data are transmitted to the receiving terminal line end.
There are also, however, systems wherein nodes that configure an ATM network are linked in a ring shape, as diagrammed in FIG. 44. In the system diagrammed here, multiple nodes 261-264 are connected by an outside ring transmission path 265 and by an inside ring transmission path 266. To each node are connected terminals A and B, i.e. 267 and 268, such as cameras or personal computers for inputting and outputting image;data, text data, and audio and video data, etc., so that, for example, image data or text data, etc., can be transmitted on the ATM network from terminal A 267 to terminal B 268. Each of the nodes here, 261-264, comprises ATM exchange with an existing STM line interface that can handle 64 Kbps, 1.5 Mbps, or 2 Mbps, etc.
In an ATM network linked by ring-shaped transmission paths such as this, ordinarily, when transmitting data from terminal A 267 to terminal B 268, one of the ring transmission paths (the outside ring transmission path 264 in the example diagrammed in FIG. 46) is used, as indicated by the broken lines in the drawing. In the event that a fault occurs in a relay transmission path or at the node 264 which is a relay node in the transmission path from the terminal A 267 to the terminal B 268, indicated by the xe2x80x9cXxe2x80x9d in FIG. 45, a loop-back is effected at the relay node 264, as indicated by the broken line in this figure, activating the transmission path to the terminal B along the stand-by inner ring transmission path 266, whereupon data can be transmitted from the terminal A to the terminal B.
However, in the event that a fault occurs at the node 261 to which the terminal B is connected, as indicated by the xe2x80x9cXxe2x80x9d in FIG. 46, loop-backs are effected at node 264 and node 262, as indicated by the broken line in FIG. 46, so that the data output by the terminal A are returned to that terminal A. Thus the fault at the terminal B 268 cannot be detected, and no alarm information is output to the terminal A. This constitutes a problem.
To deal with this problem, there is a method, called BLSR (bidirectional line switched ring), which uses loop-back switches to recover from transmission path faults that occur in ring-shaped network systems. With BLSR, line switching is performed using a so-called squelch table. This squelch table comprises node chaining information that indicates how the nodes are connected in the ring system, and squelch information indicating to which nodes signals ADDed or DROPped in line units are assigned.
FIGS. 50(a)-50(c) diagram the configuration of such a squelch table. This squelch table comprises node chaining information, cross-connect type information, squelch information, and WORK line information.
The node chaining information indicates how the nodes are connected in the ring system. The cross-connect type information indicates the channel, i.e. the type of line, used thereby. Unidirectional lines are indicated by 1WAY, bidirectional lines by 2WAY, interconnection primary nodes by 2WAYBR, interconnection (ON PROT) line primary nodes by 2WAYBRPP, and interconnection (ON PROT) line secondary nodes by 2WAYBRPS.
The squelch information indicates, in line units, ADD NODE and DROP NODE for those lines, and can set two nodes for each. When a unidirectional line (1WAY) is set, in particular, the setting is made in cognizance of the directionality of the ADD/DROP NODE lines. The WORK line information indicates which WORK line between which nodes is used by the ADDed or DROPped signal, in channel units. A protection channel (STBY) is also added as a TO channel in the configuring elements.
In the example squelch table given in FIGS. 50(a)-50(c), settings are made for the interconnection line in FIG. 48(a) using the transmission path channel 1, the interconnection (ON PROT) line in FIG. 48(b) using channel 2, and the broadcast line:in FIG. 48(c) using channel 3. This squelch table can also be used to make,settings for ordinary lines as diagrammed in FIG. 47(a).
This squelch table is indicated node by node, but the setting states are described in terms of the squelch table for node 4 represented in FIG. 51(d). In the squelch information for the TO channels 1 and 2 on the WEST side in the interconnection (ON PROT) line (channel 2), node 3 (NODE 3), a primary NODE, is set in ADDNODE (1), node 2 (NODE 2), a secondary NODE, is set in ADD NODE (2), and node 1 (NODE 1), a terminal, is set in DROP NODE 1. In the squelch information for the TO channels 1 and 2 on the EAST side, moreover, node 1 (NODE 1) is set in ADD NODE (1), node 3 (NODE 3) is set in DROP NODE (1), and node 2 (NODE 2) is set in DROP NODE (2).
The broadcast line is set in the squelch table as a combination of 1WAY lines. The purpose of this broadcast line is to prevent misconnection from occurring even when a node fault has occurred previously in node 3 or node 4, which are terminal nodes, and the DROP NODE in the squelch information sets node 2, which is the longest terminal node. Accordingly, the squelch table sets node 1 (NODE 1) in the ADD NODE (1) in the squelch information for the TO channel 3 on the EAST side, and node 2 (NODE 2), the longest terminal node, in DROP NODE (1).
As described earlier, FIGS. 48(a)-48(c) represent various line examples. The difference between the interconnection line in FIG. 48(a) and the interconnection (ON PROT) line is the difference between whether a WORK channel or a STBY channel is used for the line between node 3 and node 2. In the latter case, where node 2 does not need any particular protection, the WORK channel side is opened by using the STBY channel so that the WORK channel can be used for some other purpose, thereby enhancing line utilization efficiency. The service selector SS31 for node 3 is for selecting signals from the TORIPISUKURI and signals from the lines. The nodes line-split with this SS31 are called primary nodes, and the nodes at the end of the splits are called secondary nodes.
When a transmission path fault occurs, loop-back switching is performed on each line, but no misconnection will occur in this line example. For node faults, no misconnection will occur for one node fault, but a misconnection will occur when two nodes fault.
With reference to FIGS. 49(a)-49(c), a method for responding to a misconnection in this case is now described.
In FIGS. 49(a)-49(c), faults occur simultaneously in node 3 and node 2, whereupon a transmission path recovery process is begun using node 4 and node 1 adjacent to node 3 and node 2. Node 4 and node 1 recognize, by means of a conventional faulting node detection method, that node faults have occurred simultaneously in node 3 and node 2. Node 4 references the WORK channel squelch information in the squelch table, thereby ascertaining whether or not a path exists for communicating to the faulting node 3 and node 2. The STBY channel is used during transmission path recovery and so does not need to be retrieved.
As a consequence, in the squelch table, the TO channel discovers WEST (WORK) xe2x88x921, WEST (WORK) xe2x88x922, EAST (WORK) xe2x88x921, EAST (WORK) xe2x88x922, and EAST (WORK) xe2x88x923. If either ADD NODE (1) and ADD NODE (2), or DROP NODE (1) and DROP NODE (2) are set simultaneously, that path is used for interconnection, wherefore, when it has been verified that either ADD NODE (1) and ADD NODE (2) or DROP NODE (1) and DROP NODE (2) have experienced simultaneous node faults, this is recognized as a channel on which there is a possibility that a misconnection will occur.
That being so, in this case, WEST (WORK) xe2x88x921, WEST (WORK) xe2x88x922, EAST (WORK) xe2x88x921, and EAST (WORK) xe2x88x922 meet the condition described above, wherefore a determination is made to insert PATH AIS in the channel.
EAST (WORK) xe2x88x923 is a 1WAY path, even though node 2 is set in DROP NODE (1), so it does not become a channel subject to the insertion of PATH AIS. In terms of the actual method of PATH AIS insertion, when a faulting node is discovered in xe2x80x9cADD NODE,xe2x80x9d PATH AIS 201 is inserted for that WORK channel, and when a node fault is discovered in xe2x80x9cDROP NODE,xe2x80x9d PATH AIS 202 is inserted for the STBY channel.
In this example, accordingly, in FIGS. 51(a)-51(d), PATH AIS 301 and 302 are inserted in WEST (WORK) xe2x88x921 and WEST (WORK) xe2x88x922 and in WEST (STBY) xe2x88x921 and WEST (STBY) xe2x88x922, respectively. After this misconnection prevention processing has been completed, loop-back switching is performed and the transmission path is restored.
With the methodology disclosed in Japanese Patent Laid-open No. H5-63761/1993 (gazette) recited above, however, a protocol is required for continually verifying whether or not time information has been normally received between the transmitting and receiving ends. For this reason, the protocol provided on the transmitting and receiving ends becomes complex, which constitutes a problem.
On the other hand, when it comes to methods for performing communication connection tests, performance tests, and alarm notifications between opposing ATM exchange facilities using OAM cells, it is only possible to detect line faults between the cell resolving and assembling unit in one ATM exchange facility and the cell resolving and assembling unit in the other ATM exchange facility, and it is not possible to broaden the range of detection in order to detect whether or not a line is normal between a portion of a communication path prior to cell assembly and a portion of a communication path after cell disassembly. This constitutes a problem.
Also, the OAM cells in alarm information received from a line are transferred in a form wherein they are multiplexed with the user data (payload data) cell flow. However, because these are always multiplexed and transferred with the user data, even when there is no change in the alarm content, user data transmission,efficiency is lowered, constituting a problem.
Also, in an ATM network system that uses a ring-shaped transmission path, when a fault occurs at an opposing node, the transmission path is looped back in front of the opposing node where the fault has occurred, wherefore two problems are encountered, namely, (1) data output from a node are transmitted back at the loop-back point and returned to that same node, wherefore the fault is not detected even when the opposing equipment is.cut off, and (2), as described in FIGS. 47(a)-47(c) to FIGS. 51(a)-51(d), with the method of outputting a Pxe2x80xa2AIS to a misconnection when a loop-back is being done, logic connections for ATM cells or packets, etc., cannot be coped with.
Thereupon, an object of the present invention is to provide ATM transmission equipment that is installed between existing exchange or terminals and other carrier facilities, wherewith existing transmission paths can be emulated, and wherewith T point interfaces can be provided for in-home unit.
Another object of the present invention is to provide ATM transmission equipment wherewith the transmission bands involved in communication within an ATM network provided for STM line interface portions can be reduced and network resources can be utilized more effectively.
Another object of the present invention is to provide a line fault detection method and apparatus designed so that line faults can be detected without employing complex protocols in equipment at the transmitting and receiving ends.
Another object of the present invention is to provide a line fault detection method and apparatus designed so that normal and abnormal line conditions can be detected over a range expanded to include communication path portions prior to cell assembly and communication path portions subsequent to cell disassembly.
Another object of the present invention is to provide a line fault detection method and apparatus designed so that alarm information can be transferred while minimizing declines in user data transmission efficiency.
Another object of the present invention is to provide a line fault detection method and apparatus designed so that, even when a fault occurs in an opposing apparatus in a ring-shaped network configuration, faults in the opposing apparatus can be detected at one node, at the ATM connection/packet connection level, and that fault information output to a terminal.
In order to attain the objects stated above, ATM transmission equipment for use in a communication network wherein an exchange having a line interface for STM line is connected to digital service units through an ATM network, comprises line interface means for connecting an STM line; cell assembling means for converting a fixed-bit-rate data string on the STM line into ATM cells; cell transmission control means for transmitting the ATM cells converted by the cell assembling means to the ATM network using at least one virtual connection; and cell disassembling means for converting the It ATM cells received from the ATM network to a data string having a fixed bit rate on the STM line.
In addition, the cell assembling means and the cell disassembling means may be implemented with ATM adaptation layer type 1.
The invention may further comprise fault detection means for detecting faults in the ATM network; and abnormal signal output means for outputting abnormal signals corresponding to the faults detected by the fault detection means, in place of the data string, to either the exchange or to the digital service units.
In addition, the invention may include the fault detection means for detecting faults in the ATM network; and abnormal signal output means for outputting abnormal signals corresponding to faults detected by the fault detection means, instead of the data strings, to either the exchange or to the digital service unit.
The data string may contain information portions, alarm signals, and other control frames on the STM line.
ATM transmission equipment for use in a communication network wherein an exchange having a line interface for STM line is connected to terminals having user network interfaces for an STM network through an ATM network, comprises line interface means for connecting an STM line; cell assembling means for converting a fixed-bit-rate data string on the STM line into ATM cells; cell transmission control means for transmitting the ATM cells converted by the cell assembling means to the ATM network using at least one virtual connection; and cell disassembling means for converting the ATM cells received from the ATM network to a data string having a fixed bit rate on the STM line.
The cell assembling means and the cell disassembling means may be implemented with ATM adaptation layer type 1.
In addition, the invention may include fault detection means for detecting faults in the ATM network; and abnormal signal output means for outputting abnormal signals corresponding to the faults detected by the fault detection means, in place of the data string, to either the exchange or to the terminals.
The fault detection means detects faults in the ATM network based on outputs from the cell disassembling means.
The data string may contain information portions, alarm signals, and other control frames on the STM line.
ATM transmission equipment installed in an ATM network in a communication network configured by inserting the ATM network between an exchange having an STM line interface and digital service units, comprising line interface means for connecting an STM line; cell assembly/disassembly means for performing process of converting a data string on the STM line to ATM cells and process of disassembling the ATM cells; alarm signal detection means for performing processing for either detecting communication anomalies or extracting maintenance operation information from the data string on the STM line; transmission means for replacing only necessary information resulting from detection or extraction by the alarm signal detection means with alarm signals, and transmitting that information together with information channels in the data string through the cell assembly/disassembly means; and data rearranging means for disassembling ATM cells formed from the information channels and the alarm information, rearranging those data into data string for the STM line, and outputting the data string either to the exchange or to the digital service unit.
In addition, the invention may include fault detection means for detecting faults in the ATM network; and fault information insertion means for inserting fault information indicating fault detection results in place of prescribed data in the data string when rearrangement to the data string is done by the data rearranging means.
The fault detection means detects faults in the ATM network either by anomalies in the cell disassembly process in the cell assembly/disassembly means or by anomalies in data on channels that transmit the alarm signals.
The cell assembly/disassembly means merges the alarm signals with the information channels and forms cells using the same virtual connection.
The cell assembly/disassembly means forms the alarm signals and the information channels, respectively, into cells using different virtual connections.
The cell assembly/disassembly means may perform the cell assembling and the cell disassembling in accordance with provisions of ATM adaptation layer type 1.
ATM transmission equipment installed in an ATM network in a communication network configured by inserting the ATM network between an exchange having an STM line interface and in-home unit having a user-network interface for the STM line, comprising line interface means for connecting the STM line; user-network interface means having a user-network interface corresponding to the STM line; cell assembly/disassembly means for performing process of converting data string on the STM line to ATM cells and process of disassembling the ATM cells; alarm signal detection means for performing processing for either detecting communication anomalies or extracting maintenance operation information from data string on the STM line; transmission means for replacing only necessary information resulting from detection or extraction by the alarm signal detection means with alarm signals, and transmitting that information together with information channels in the data string through the cell assembly/disassembly means; and data rearranging means for disassembling ATM cells formed from the information channels and the alarm information, rearranging these data into a data string for the STM line, and outputting the data string either to the exchange or to the digital service unit.
The invention may include fault detection means for detecting faults in the ATM network; and fault information insertion means for inserting fault information indicating fault detection results in place of prescribed data in the data strings when rearrangement to the data strings is done by the data rearranging means.
The fault detection means detects faults in the ATM network either by anomalies in cell disassembly process in the cell assembly/disassembly means or by anomalies in data on channels that transmit the alarm signals.
The cell assembly/disassembly means may merge the alarm signals with the information channels and form cells using same virtual connection.
The cell assembly/disassembly means may also form the alarm signals and the information channels, respectively, into cells using different virtual connections.
The cell assembly/disassembly means may perform the cell assembly and the cell disassembly in accordance with provisions of ATM adaptation layer type 1.
A method in a communication system that assembles data input from a line interface into data of fixed bit length or data of variable bit length and transmits the data thus assembled to a transmitting destination, comprising the steps of, in an apparatus at transmitting end, separating data input from the line interface into payload portions and alarm data portions; adding error detection codes to the alarm data portions; and assembling the alarm data portions having the added error detection codes and the payload portions into fixed-length data in the same transmitting unit and transmitting the data thus assembled to transmitting destination, and, in an apparatus at receiving end, separating the fixed-length data or the variable-length data received from the transmitting end into payload portions and alarm data portions; detecting whether or not errors are contained in the alarm data portions so separated by the error detection codes added to the alarm data portions; assembling the received payload portions and the alarm data portions are synthesized into a frame of a line interface, and the frame thus synthesized are transmitted to a line interface at the receiving end, if there are no errors in the alarm data portions; and determining that a line fault has developed, and transmitting line-break alarm data to the line interface at the receiving end, if there are errors in the alarm data portions.
When an error in an alarm data portion continues at or above some preset value, the line-break alarm data are transmitted, and when the preset value is not reached, last data wherein there was no error are transmitted to the line interface at the receiving end.
When underflows are detected in a receiving buffer that sequentially stores the fixed-length data or the variable-length data received from the transmitting end; and, when an underflow is detected that extends beyond a preset time, the line-break alarm data are transmitted to the line interface at the receiving end.
The alarm data portions and the payload portions transmitted from the apparatus at the transmitting end may be transmitted with different connections.
The alarm data portions may be transmitted to the line interface at the receiving end only when there is a change in alarm content.
The formula for calculating the error detection codes added to the alarm data portions is made to differ according to whether direction of bidirectional path for the payload portions is upstream or downstream.
An initial value of formula for calculating the error detection codes added to the alarm data portions is made to differ according to whether direction of bidirectional path for the payload portions is upstream or downstream.
A method in a communication system that assembles data input from a line interface into fixed-length data and transmits the data thus assembled toward a transmitting destination, comprising the steps of providing a test data generator and a test data comparator in a line terminal portion in a line interface unit; generating test data by the test data generator, either prior to starting operation of the communication system or when a fault occurs during operation thereof; transmitting the test data via an established line toward a line terminal portion in an opposing line interface unit; comparing the test data so transmitted against test data that return via the opposing line terminal portion by the test data comparator; and detecting presence or absence of anomalies in line leading to the line terminal portion in the opposing line interface unit according to whether or not those data agree.
The line interface unit is provided in an ATM exchange that input and outputs non-ATM data such as video and audio data.
An alarm information transfer method in a communication system that assembles data input from a line interface into fixed-length data and transmits the data thus assembling toward a transmitting destination, when transmitting alarm information indicating a line fault detected at receiving end to opposing apparatus, comprising the steps of assembling information into data of fixed bit length; and transmitted toward opposing transmitting destination, only when there is a change in alarm content.